1. Field of the Invention
The present invention relates to apparatus for lifting well fluids from a subterranean reservoir. More specifically, the present invention pertains to gas operated plunger apparatus for lifting well fluids from a subterranean reservoir through a well conduit which extends to the surface. In particular, the present invention pertains to plunger apparatus which is suitable for lifting well fluids from a subterranean reservoir with relatively low gas pressures.
2. Description of the Prior Art
Gas operated plungers or pistons, have, for many years been utilized in producing subterranean wells where the natural gas pressure in the well is insufficient to produce a free flow of liquids to the surface. Such prior art devices generally include a plunger or piston element which is slidably and sealably engaged with a well conduit which extends upwardly from the well reservoir to the surface. The well conduit, normally referred to as a production string, may be either a casing string or a tubing string disposed within a casing string.
Typically, a gas operated plunger is provided with external sealing elements for sealingly and slidingly engaging the inner walls of the well conduit. However, the plunger is also typically provided with an internal passage which bypasses the external sealing elements and allows the plunger to gravitate or fall from the surface of the well to the reservoir. A valve mechanism is typically provided within the body of the plunger and is effective to close the internal passage through the plunger when the plunger has reached a preselected depth in the well. This preselected position may be determined by a stop device, sometimes referred to as a tubing stop, which is semi-permanently located in the well conduit. The tubing stop may be relocated, from the surface of the well, at different depths as well conditions change.
When the plunger reaches the tubing stop, contact with the stop causes the valve mechanism to close, blocking the internal passage through the plunger. The plunger remains in this position while well fluids accumulate above the plunger and gas pressure begins to build. After a preselected time, a control valve at the surface of the well opens, allowing flow of fluids to commence. The drop in pressure above the plunger allows higher pressures below the plunger to force it and the well fluids thereabove to the surface. On arrival of the plunger at the surface, the plunger valve opens, opening the internal flow passage through the plunger, and the control valve is closed. This allows the plunger to gravitate to the bottom of the well conduit until stopped by the tubing stop. The plunger valve is again closed and the next cycle begins. Examples of such gas operated plunger apparatus may be seen In U.S. Pat. Nos. 4,070,134 and 4,923,372.
Most gas operated plungers of the prior art have inherent problems. One problem is with the seal between the plunger and the inner walls of the well conduit. This seal must be capable of sliding past restrictions within the well conduit yet slidingly and sealingly engageable with the inner walls of the conduit sufficiently to provide the pressure differential necessary for elevating the plunger and well fluids thereabove to the surface of the well and capable of doing so without undue damage to the seal. This type of seal frequently comprises a plurality of metallic pads or segmented cylindrical surfaces provided on a plurality of longitudinally separated cylindrical segments surrounding the plunger body. These pads or segments are radially moveable between an innermost position and an outermost position so that the seal may contract as it passes through restricted portions of the well conduit and may expand to sealingly and slidingly engage the well conduit to provide the necessary seal. Typically, some type of biasing means is provided to bias the sealing elements or pads toward their outermost positions. However, when the pads are in the outermost expanded positions a considerable amount of leakage and bypassing occurs between the interior surfaces of the pads and the plunger body. This, of course, increases the pressure necessary to elevate the plunger and well fluids from the subterranean formation to the surface of the well. Furthermore, this flow and leakage by the seal elements and the plunger body creates erosion that will erode the plunger body and the sealing elements, further increasing the pressure necessary to elevate the plunger and well fluids to the surface of the well and eventually eroding these elements to such an extent that they are no longer useable, requiring repair and/or replacement at considerable expense. In addition, in the plunger apparatus designs of the prior art, the seals inherently leak through the spaces between adjacent segments or pads. These spaces are wider and leak more as the internal diameter of the well conduit increases, as is Frequently the case with the upper portions of a well conduit. This type of leakage increases the pressure necessary for lifting of the plunger and well fluids, requires more plunger trips, and also results in erosion of the sealing elements or pads and plunger body so that the sealing elements and/or the plunger body may eventually have to be replaced.
In many gas operated plungers of the prior art, particularly those in current use, a rod is concentrically mounted in the internal flow passage therethrough. The purpose of the rod is to reopen the plunger valve when it reaches the surface of a well. Typically, the rod would engage a spring loaded stop at the surface of the well causing the rod to move downwardly to engage a valve closure member, unseating the closure member and opening the valve. Such a design severely restricts flow through the internal flow passage as the plunger is falling back to the bottom of the well. In some conditions, it restricts the flow to such an extent that the plunger will not fall without added weight. This type of design is costly, requires corrections for the additional moving weight and creates other hazards for the plunger as it returns to the surface. Fluid flow through the plunger should be maintained at an uninterrupted maximum. If not, the plunger will fall erratically and may not reach the depth required to close the plunger valve. If the valve does not close, the plunger will not return to the surface of its own accord, requiring a costly fishing operation to retrieve the plunger. In addition to the above mentioned problems, the rod provided to the valve is necessarily small, fragile and susceptible to damage.
Some designs have attempted to solve the problems associated with a rod in the internal flow passage of the plunger by providing a rod in a lubricator bumper sub typically installed at the upper end of the well conduit. Such a rod is designed to penetrate the plunger flow passage to contact the valve closure member and push it off its seat at the instant the plunger contacts a spring loaded stop in the lubricator bumper sub. This requires accurate and sensitive adjustments which are not always possible in field situations. Furthermore, there are many hazards and damage possibilities when the plunger, traveling under pressure, is propelled against a stationary rod. There are so many disadvantages of this design, that very few well operators utilize it.